European Patent Application (OPI) No. 47005 describes 9-fluoro-10-substituted-3-methyl-7-oxo-2,3-dihydro-7H-pyrido(1,2,3-de)-1,4 -benzoxazine-6-carboxylic acid and West German Patent Application (OPI) No. 2914258 describes 9-fluoro-8-(4-methyl-1-piperazinyl)-5-methyl-6,7-dihydro-1-oxo-1H,5H-benz( ij)quinolizine-2-carboxylic acid (the term "OPI" as used herein refers to a "published unexamined application").
The present inventors found that introduction of a methylene group (.dbd.CH.sub.2) into the tricyclic compounds brought the excellent antibacterial activity and completed this invention.